Physiological and Ecological Responses of Photosynthetic Processes to Oceanic Properties and Phytoplankton Communities in the Oligotrophic Western Pacific Ocean

Yuqiu Wei; Zhuo Chen; Congcong Guo; Qi Zhong; Chao Wu; Jun Sun

doi:10.3389/fmicb.2020.01774

Physiological and Ecological Responses of Photosynthetic Processes to Oceanic Properties and Phytoplankton Communities in the Oligotrophic Western Pacific Ocean

Front Microbiol. 2020 Aug 4:11:1774. doi: 10.3389/fmicb.2020.01774. eCollection 2020.

Authors

Yuqiu Wei¹, Zhuo Chen^{2

3}, Congcong Guo¹, Qi Zhong¹, Chao Wu^{2

3}, Jun Sun^{2

3}

Affiliations

¹ Institute of Marine Science and Technology, Shandong University, Qingdao, China.
² Research Centre for Indian Ocean Ecosystem, Tianjin University of Science and Technology, Tianjin, China.
³ Tianjin Key Laboratory of Marine Resources and Chemistry, Tianjin University of Science and Technology, Tianjin, China.

Abstract

Understanding the dynamics of primary productivity in a rapidly changing marine environment requires mechanistic insight into the photosynthetic processes (light absorption characteristics and electron transport) in response to the variability of environmental conditions and algal species. Here, we examined the photosynthetic performance and related physiological and ecological responses to oceanic properties [temperature, salinity, light, size-fractionated chlorophyll a (Chl a) and nutrients] and phytoplankton communities in the oligotrophic Western Pacific Ocean (WPO). Our results revealed high variability in the maximum (F_v/F_m; 0.08-0.26) and effective (F_q'/F_m'; 0.02-0.22) photochemical efficiency, the efficiency of charge separation (F_q'/F_v'; 0.19-1.06), the photosynthetic electron transfer rates (ETR_RCII; 0.02-5.89 mol e^- mol RCII^-1 s^-1) and the maximum of primary production [PP_max; 0.04-8.59 mg C (mg chl a)^-1 h^-1]. All these photosynthetic characteristics showed a depth-specific dependency based on respective nonlinear regression models. On physiological scales, variability in light absorption parameters F_v/F_m and F_q'/F_m' notably correlated with light availability and size-fractionated Chl a, while both ETR_RCII and PP_max were correlated to temperature, light, and ambient nutrient concentration. Since the presence of nonphotochemical quenching (NPQ_NSV; 2.33-12.31) and increasing reductant are used for functions other than carbon fixation, we observed nonparallel changes in the ETR_RCII and F_v/F_m, F_q'/F_m', F_q'/F_v'. In addition, we found that the important biotic variables influencing F_v/F_m were diatoms (cells > 2 μm), picosized Prochlorococcus, and eukaryotes, but the PP_max was closely related to large cyanobacteria (cells > 2 μm), dinoflagellates, and picosized Synechococcus. The implication is that, on ecological scales, an interaction among temperature, light, and nutrient availability may be key in driving the dynamics of primary productivity in the WPO, while large cyanobacteria, dinoflagellates, and picosized Synechococcus may have a high contribution to the primary production. Overall, the photosynthetic processes are interactively affected by complex abiotic and biotic variables in marine ecosystems, rather than by a single variable.

Keywords: Western Pacific Ocean; oceanic properties; photosynthesis; phytoplankton; primary production.